This invention relates to the processing of titanium alloy articles fabricated by powder metallurgy to improve the microstructure of such articles.
Titanium alloy parts are ideally suited for advanced aerospace systems because of their excellent general corrosion resistance and their unique high specific strength (strength-to-density ratio) at room temperature and at moderately elevated temperatures. Despite these attractive features, the use of titanium alloys in engines and airframes is often limited by cost due, at least in part, to the difficulty associated with forging and machining titanium.
To circumvent the high cost of titanium alloy parts, several methods of making parts to near-net shape have been developed to eliminate or minimize forging and/or machining. These methods include superplastic forming, isothermal forging, diffusion bonding, investment casting and powder metallurgy, each having advantages and disadvantages.
Until relatively recently, the primary motivation for using the powder metallurgy approach for titanium was to reduce cost. In general terms, powder metallurgy involves powder production followed by compaction of the powder to produce a solid article. The small, homogeneous powder particles provide a uniformly fine microstructure in the final product. If the final article is made into a net-shape by the application of processes such as Hot Isostatic Pressing (HIP), a lack of texture can result, thus giving equal properties in all directions. The HIP process has been practiced within a relatively broad temperature range, for example, about 700.degree. to 1200.degree. C. (1300.degree.-2200.degree. F.), depending upon the alloy being treated, and within a relatively broad pressure range, for example, 1 to 30 ksi, generally about 15 ksi.
Recent developments in advanced hypersonic aircraft and propulsion systems require high temperature, low density materials which allow higher strength to weight ratio performance at higher temperatures. As a result, titanium aluminide alloys are now being targeted for many such applications. Titanium aluminide alloys based on the ordered alpha-2 Ti.sub.3 Al phase are currently considered to be one of the most promising group of alloys for this purpose. However, because of its ordered structure, the Ti.sub.3 Al ordered phase is very brittle at lower temperatures and has low resistance to cracking under cyclic thermal conditions. Consequently, groups of alloys based on the Ti.sub.3 Al phase modified with beta stabilizing elements such as Nb, Mo and V have been developed. These elements can impart beta phase into the alpha-2 matrix, which results in improved room temperature ductility and resistance to thermal cycling. However, these benefits are accompanied by decreases in high temperature properties. With regard to the beta stabilizer Nb, it is generally accepted in the art that a maximum of about 11 atomic percent (21 wt %) Nb provides an optimum balance of low and high temperature properties.
Currently, Nb-modified Ti.sub.3 Al alloys offer improvements in both hot workability and room temperature ductility as a result of grain refinement, increased slip capabilities in the beta phase, and reduction of the beta-transus temperature. Rapid solidification of these alloys offers the potential for improvement in ductility by grain refinement, by increased alloying possibilities, and by enhanced disordering of the alpha-2 phase. Titanium aluminide alloys can be processed economically utilizing a powder metallurgy (PM) route to produce a near net shape (NNS).
Accordingly, it is an object of the present invention to provide a process for producing articles having a desirable fine microstructure by powder metallurgy of titanium aluminide alloys.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art after reading the detailed description of the invention as well as the appended claims.